1. Field of the Invention
The present invention relates to a device structure suitable for transistors, thyristors, optical emitters, optical detectors, and other solid state devices.
2. Description of the Prior Art
The speed of operation and transconductance are two properties of transistors that device designers are endlessly seeking to increase, in order to obtain superior performance. Generally speaking, from the first practical implementation of the bipolar and field effect transistors in the 1950's and early 1960's, the major advances in device performance have resulted from technological inovation. The principles of operation have remained unchanged, but the physical sizes of the devices have been reduced at a steady rate towards smaller geometries. The progression towards miniaturization has been due to advances in lithography, processing technology, and improvement in device structural design.
Initially the bipolar transistor was the superior high-speed device, while the field effect transistor (FET) was more useful for low-speed/low-power applications. With continued scaling however, the FET has shown the ability to outperform bipolar transistors for very high speed and very high density circuits. The field effect transistor is frequently implemented as a metal-oxide semiconductor (MOS) structure, although other forms are possible. The advantages of the bipolar vis-a-vis the MOS are its higher transconductance (g.sub.m) and its higher current gain. However, to compare logic circuit performance one also has to consider the ratio of transconductance to capacitance (g.sub.m /C). In this regard, MOS technology benefits distinctly compared to bipolar technology as device geometries are reduced, because in MOS devices, capacitances are more amenable to scaling. This is because the shrinking of vertical dimensions in the MOS (junction depth and oxide thickness) is more easily accomplished than in the bipolar (emitter depth and base width).
The endless thrust to ever-decreasing device geometries leads to problems in both bipolar and field effect transistors, with one serious problem encountered by either device being the punchthrough effect. In the bipolar transistor, punchthrough occurs when the collector merges with the emitter, and in the field effect transistor when the drain and source depletion regions begin to merge. In both cases the characteristics change from the pentode-like to the triode-like form, which degrades inverter performance and eventually leads to complete loss of control of the base or gate, as the case may be.
For the last 25 years bipolar and field effect transistors have been the dominant device forms because of their proven abilities, although from time-to-time various other devices have been proposed. In particular, considerable interest has been shown in hot-electron devices--e.g., the tunnel-transistor (or MOM OM), the semiconductor-metal-semiconductor transistor, and the space-charge-limited transistor--because of their potential for high-speed operation. However, for various reasons, none of these devices have proved viable. Nevertheless, the basic concept of a device operating in the collision-free mode has its attractions.
Since the limits of performance of field effect and bipolar devices are looming ominously closely, there is a pressing need to consider new concepts to circumvent the basic limits of these devices. Any new device would desirably have at least some of the following characteristics:
I. have a transconductance and current gain equal to, or better than, the conventional bipolar transistor; PA1 II. have a figure of merit (g.sub.m /C) better than state-of-the-art bipolar or field effect transistors; PA1 III. be immune to punchthrough effects; PA1 IV. be integrable to a useful level of circuit complexity, PA1 V. permit fabrication of logic and memory circuits with power/delay products superior to state-of-the-art circuits; PA1 VI. be able to attain transit times that are not restricted solely by the lithographic capability; PA1 VII. be free of the scaling limitations on existing devices.
Thus, it is desirable to obtain a device that combines the virtues of the bipolar and MOS concepts; namely one which has high current gain and high transconductance coupled with relatively simple fabrication requirements and low charge storage.